Display devices are used in various types of electronic devices such as monitors, laptop computers, TVs and mobile communication terminals. For such devices, it is desirable for the display to be thin and/or light. To satisfy such requirements, various Flat Panel Displays (FPDs) have been developed as alternatives to more conventional cathode ray tube displays.
An LCD (Liquid Crystal Display) is one kind of Flat Panel Display. In an LCD, dielectric anisotropic liquid crystal is injected between an upper substrate's alignment layer having a common electrode and a lower substrate's alignment layer having a pixel electrode. An image is displayed by applying voltage into the common electrode and the pixel electrode to form an electric field between the common electrode and the pixel electrode, and then controlling intensity of the electric field to control transmittance of light of the liquid crystal.
RGB (Red, Green, Blue) data are input into the LCD from an external host system, e.g., a graphic source. The data format of the input RGB data is converted by a timing controller of the LCD, and transmitted to a source driver IC (Integrated Circuit). The source driver IC generates an analog gray scale voltage corresponding to the RGB data and supplies the analog signal to selected pixels of the LCD panel, to thereby cause the LCD panel to display an image.
Generally, the bit number of the RGB data input into the timing controller needs to be the same as that of the data signal of the source driver IC. In general, a color depth of 18-bits (i.e. each of the Red, Green and Blue data is 6 bits (n=6)), or 24-bits (i.e. each of the Red, Green and Blue is 8 bits (n=8)) is commonly used in LCDs.
As electrical devices including LCD displays have become bigger, a source driver IC capable of handling a data signal having more than 10 bits (n=10) so as to display a broader range of colors has been desired.
However, increasing the number of data bits of a source driver IC may be difficult and/or expensive. In particular, a digital-to-analog converter for converting input digital data to analog gray scale voltage is built in the source driver IC. Because the number of transistors included in a digital-to-analog converter increases greatly according as the number of data bits increases, the more the number of data bits is increased, the more the size of a conventional source driver IC chip must be increased.
FIG. 1 is a circuit diagram illustrating a conventional digital-to-analog converter having a full-type decoder.
As illustrated in FIG. 1, the digital-to-analog converter includes a gamma reference voltage generating circuit 10 which generates gamma reference voltages having a plurality of levels and a decoder 20 which receives 8-bit digital data to select one of the gamma reference voltages based on the 8-bit digital data.
The gamma reference voltage generating circuit 10 includes a plurality of resistor arrays connected in series between the GVDD and the VGS voltage levels, and generates 256 gamma reference voltages having 256 voltage levels using a voltage divider such as the resistor arrays.
Although not illustrated, the gamma reference voltage generating circuit 10 may include a gamma compensation circuit that can compensate for the gamma reference voltages so that the compensated gamma reference voltages may substantially conform to an ideal gamma curve.
The decoder 20 receives 8-bit digital data, i.e. bit values D0, D1, D2, D3, D4, D5, D6 and D7 and inverted values of each of the bit values, i.e. D0B, D1B, D2B, D3B, D4B, D5B, D6B and D7B, and selects one gamma reference voltage from among V0, V1, V2, . . . , V254, V255 gamma reference voltages generated by the gamma reference voltage generating unit 10 based on the value of the 8-bit digital data.
The decoder 20 includes 256 MOS transistor arrays 21 respectively corresponding to V0, V1, V2, . . . , V254, V255 gamma reference voltages generated from the gamma reference voltage generating unit 10. Each of the 256 MOS transistor arrays 21 includes eight MOS transistors connected in series, and each of the eight MOS transistors corresponds to a bit of input digital data. The corresponding bit values of the input digital data, or the inverted values, are input into the gate of each of the eight MOS transistors.
For example, in case where the input digital data are ‘0000001’, a gamma reference voltage V1 will be output, because the gate of MOS transistor M0 is connected to D0, while all of the MOS transistors M1 through M7 are connected to the inverted signals D1B to D7B, respectively, in a second MOS transistor array 21 that is configured to select the gamma reference voltage V1. The output gamma reference voltage V1, i.e. an analog gray scale voltage, is amplified to have predetermined voltage level, and then, input into an LCD panel (not shown).
However, in cases where an 8-bit module decoder is implemented as a full-type decoder, 256 (i.e. 28) MOS transistor arrays 21 are required in order to be able to select each of the 256 gamma reference voltages. As a result, a switching circuit may require a large number (8×256=2048) of MOS transistors. Therefore, it is difficult to miniaturize the chip size, and the circuit may consume excessive power.
Furthermore, configuring a decoder 20 for decoding 10-bit digital data requires 1024 MOS transistor arrays respectively having 10 MOS transistors in order to be able to select one of 1024 gamma reference voltages, so that 10×1024=10240 MOS transistors may be required. Thus, the number of the MOS transistors needed for a 10-bit decoder is increased more than 4 times compared with a decoder for decoding 8-bit digital data.
Consequently, such a full-type decoder selects one among the 2n input gamma reference voltages to decode n-bit digital data, so that the full-type decoder can output accurate analog gray scale voltage. However, a full-type decoder may require a very large chip area in order to decode 10-bit digital data. In addition, a full-type decoder may consume a significant amount of power. Because the decoder may occupy almost 50 percent of the chip area in a source driver IC, when the size of the decoder increased, it may not be possible to minimize the chip size.
Because of these problems, fractional decoders such as a half-type decoder and a quarter-type decoder have been used. A fractional decoder outputs a number of analog gray scale voltages by using two gamma reference voltages generated from a gamma reference voltage generating unit. As will be explained in greater detail below, an averaging amplifier coupled to the fractional decoder generates a weighted average of the two gamma reference voltage to provide gamma reference voltages intermediate and/or equal to the two gamma reference voltages. The half-type decoder and the quarter-type decoder can reduce the number of MOS transistors to one half or one fourth, respectively, compared with the full-type decoder using all the 2n gamma reference voltages.
FIG. 2 is a circuit diagram illustrating a conventional 10-bit standard half-type decoder used in a source driver IC.
As illustrated in FIG. 2, a half-type decoder 80 includes a gamma reference voltage selecting unit 40 and an averaging amplifier 50. The gamma reference voltage selecting unit 40 receives V0, V2, V4, V6, . . . from a gamma reference voltages generating unit, and receives 10-bit digital data, i.e. D0, D1, D2, D3, D4, D5, D6, D7, D8 and D9 bit value and the inverted values of each bit value, i.e. D0B, D1B, D2B, D3B, D4B, D5B, D6B, D7B, D8B and D9B, for selecting the gamma reference voltage, and outputs two gamma reference voltages Y1 and Y2. The averaging amplifier 50 receives two gamma reference voltages output from the gamma reference voltage selecting unit 40, and outputs an average voltage of the two.
FIG. 3 is a table illustrating functioning of half-type decoder 80 illustrated on FIG. 2.
Referring to FIG. 2 and FIG. 3, the half-type decoder 80 selects and outputs two gamma reference voltages Y1 and Y2, by using a predetermined gamma reference voltage and a gamma reference voltage 2 gray scale levels higher than the predetermined gamma reference voltage, and then, outputs an average voltage Ya of the two gamma reference voltages Y1 and Y2.
For example, in case where the input digital data are ‘0000000000’, because the gamma reference voltage selecting unit 40 selects V0 and outputs V0 as both Y1 and Y2, an average value V0 is output through the averaging amplifier 50.
In addition, in case where the input digital data are ‘0000000001’, because the gamma reference voltage selecting unit 40 selects V0 and V2 and outputs V0 and V2 as Y1 and Y2, respectively, an average value V1 of V0 and V2 is output through the averaging amplifier 50 as output Ya. In addition, in case where the input digital data are ‘0000000010’, because the gamma reference voltage selecting unit 40 selects and outputs V2 and V2 as Y1 and Y2, respectively, an average value Ya of V2 is output by the averaging amplifier 50.
In this manner, the half-type decoder 80 can choose all of the analog gray scale voltages to be input to a LCD panel by using only 512 gamma reference voltages among 1024 gamma reference voltages. Therefore, because the number of the MOS transistor arrays for selecting each of the gamma reference voltages can be greatly reduced, the area of the chip and the dissipated power can be reduced by almost one half compared with the full-type decoder.
FIG. 4 is a circuit diagram illustrating a conventional 10-bit standard quarter-type decoder used in a source driver IC.
As illustrated in FIG. 4, a quarter-type decoder 90 includes a gamma reference voltage selecting unit 60 and an averaging amplifier 70. The gamma reference voltage selecting unit 60 receives the gamma reference voltages, i.e. V0, V4, V8, V12, . . . from the gamma reference voltages generating unit 10, and receives 10-bit digital data, i.e. bit values D0, D1, D2, D3, D4, D5, D6, D7, D8 and D9 and inverted values of each of the bit values, i.e. D0B, D1B, D2B, D3B, D4B, D5B, D6B, D7B, D8B and D9B for selecting and outputting one of the gamma reference voltages, and outputs four gamma reference voltages Y1–Y4. The averaging amplifier 70 receives four gamma reference voltages Y1–Y4 output from the gamma reference voltage selecting unit 60, and outputs an average voltage Ya.
FIG. 5 is a table illustrating the operation of a quarter-type decoder 90 illustrated in FIG. 4.
Referring to FIG. 5, the half-type decoder 90 selects and outputs four gamma reference voltages, i.e. Y1, Y2, Y3 and Y4 by using a given gamma reference voltage and a gamma reference voltage 4 gray scale levels higher than the given gamma reference voltage, and then, outputs average voltage Ya of Y1, Y2, Y3 and Y4.
For example, in case where the input digital data are ‘0000000000’, because the gamma reference voltage selecting unit 60 selects and outputs V0, V0, V0 and V0 as Y1–Y4, an average value V0 is output as Ya by the averaging amplifier 70.
In addition, in case where the input digital data are ‘0000000001’, because the gamma reference voltage selecting unit 60 selects V0, V0, V0 and V4 and outputs V0, V0, V0 and V4 as Y1–Y4, an average value V1 is output as Ya by the averaging amplifier 70. In addition, in case where the input digital data are ‘0000000010’, because the gamma reference voltage selecting unit 60 selects V0, V0, V4 and V4 and outputs V0, V0, V4 and V4 as Y1–Y4, Ya equals V2 when average value is output through the averaging amplifier 70.
In case where the input digital data are ‘0000000011’, because the gamma reference voltage selecting unit 60 selects V0, V4, V4 and V4 and outputs V0, V4, V4 and V4 as Y1–Y4, Ya equals V3 when the average value is output by the averaging amplifier 70. In addition, in case where the input digital data are ‘0000000100’, because the gamma reference voltage selecting unit 60 selects V4, V4, V4 and V4 and outputs V4, V4, V4 and V4 as Y1–Y4, Ya equals V4 when an average value is output by the averaging amplifier 70.
In this manner, the quarter-type decoder 90 can generate 1024 analog gray scale voltages to be input to an LCD panel by using only 256 gamma reference voltages. Therefore, because the number of MOS transistor arrays for selecting each of the gamma reference voltages can be greatly reduced, the area of the chip and the dissipated power can be reduced compared with a full-type decoder and/or a half-type decoder.
However, while such a half-type decoder and/or a quarter-type decoder can reduce the area of a chip used for the source driver IC and/or the power dissipation thereof, it may be difficult to output accurate analog gray scale voltage because the gamma reference voltage input to the decoder may not be linear over the entire range of gamma reference voltages.
FIG. 6 is a graph illustrating a gamma curve of gamma reference voltage. The y-axis represents brightness and the x-axis represents gamma reference voltage.
Referring to FIG. 6, because the gamma reference voltage is nonlinear in certain regions, such as in the vicinity of the beginning section V0˜V7 and in the vicinity of the last section V1016˜V023, and because, in the nonlinear intervals (a, c), the average voltage of 2 (or 4) gamma reference voltages selected by a given gamma reference voltage and a gamma reference voltage 2 gray scale levels (or 4 gray scale levels) higher than the given gamma reference voltage may not be exactly equal to a gamma reference voltage located between the two gamma reference voltages (i.e. the given gamma reference voltage and a gamma reference voltage 2 gray scale levels higher than the given gamma reference voltage), accurate analog gray scale voltages may not be output. Consequently, a distortion of the gamma curve occurs, and the display quality may be deteriorated.
While a conventional full-type decoder can output a relatively accurate analog gray scale voltage, such a circuit may require a large chip area and/or may have a large power dissipation. Moreover, while a conventional half-type decoder or a conventional quarter-type decoder may require less chip area and may have reduced power dissipation, such circuits may not output accurate analog gray scale voltage levels in the nonlinear intervals (a, c) of the gamma reference voltage curve.